1. Field of the Invention
This application relates to a device whereby the frequency and phase of a laser is actively stabilized by locking to a transient spectral hole in a solid state or other condensed phase reference material.
2. Description of the Related Art
Lasers emit electromagnetic radiation characterized by the optical range of the spectrum where wavelengths are expressed in nanometers (nm) corresponding to 10xe2x88x929m, and frequencies are expressed in megaHertz (MHz) corresponding to 106 Hz, or gigaHertz (GHz) corresponding to 109 Hz, or teraHertz (THz) corresponding to 1012 Hz, where one Hz is one cycle per second.
Previous laser stabilization techniques have relied on frequency references based on Fabry-Perot interferometer resonances or the center frequencies of transitions in atomic or molecular vapors.
In the prior art, for example, the Pound-Drever-Hall laser frequency locking technique utilizes frequency modulated (FM) spectroscopy techniques to actively lock the laser frequency to a reflection mode of a Fabry-Perot interferometer.
Use of an error signal to stabilize a laser frequency of a tunable laser according to the Pound-Drever-Hall technique is shown in FIGS. 1A and 1B. In FIG. 1A a laser 110 emits a beam along path 161 impinging on a phase modulator 112. The radio frequency (rf) signal generator 142 supplies an electrical signal transmitted by wire to the modulator 112 and to mixer 140. The modulator 112 modulates and passes a first beam portion along path 162 to a beam splitter 114. Beam splitter 114 passes a second beam portion along a beam path 163 to a cavity, 116 between mirrors 115 and 117, usually enclosed in a vacuum chamber and extremely well isolated from vibrations. The cavity 116 is tuned to a reference frequency by choice of spacing between mirrors 115 and 117 and the cavity stores and re-emits a reference beam having an energy peak at the reference frequency along path 163 to splitter 114. At the beam splitter 114 both the modulated beam, which reflects immediately off mirror 115, and the reference beam are diverted along path 165 to a detector 120 which outputs a detector electrical signal. The detector reference signal contains rf frequency components generated by the interference between the modulated beam and the cavity reference beam. This detector electrical signal is filtered at filter 130 to remove unwanted harmonic frequencies and to generate an error signal that is output to the mixer 140. At mixer 140, the rf signal from the rf signal generator 142 is combined with the error signal at rf frequencies to demodulate the error signal to lower frequencies where it is used as a control signal that is output to the laser servo electronics 150. The servo electronics 150 tune the laser 110 in response to the control signal received.
The required frequency modulation, achieved in the above realization by use of the modulator 112, also may be achieved by other means, such as current modulation of the laser diode driver current under control of the radio frequency (rf) signal generator 142, when a laser diode is used as the optical source 110. For other lasers, other equivalent means of modulation also may be understood.
FIG. 1B shows the output 125 of cavity transmission detector 122 and the error signal 145 output (showing one example, from a class of possible frequency modulation quadrature signals) by error signal detector 120 combined with the mixer 140 which is sent to the servo electronics 150 as the modulated laser frequency beam 162 is tuned across the cavity reference frequency. The amplitude of the error signal 145 at a given frequency in the vicinity of the lock point 147 is related to the size and direction of the deviation of frequency from the reference frequency at the lock point 147. Thus the greater the deviation of the laser frequency from the reference frequency, the greater is the error signal and the greater is a component of the input to the servo electronics 150 to tune the laser back toward the reference frequency at the lock point 147. The stability of the laser frequency is limited by the characteristics of the peak in the cavity transmission signal used as the reference frequency detected by the detector 120 and the quality of the entire feedback system of FIGS. 1A and 1B.
When quoting the degree of stability of a laser, one simultaneously quotes the degree of stabilization and the time period of observation.; the two are correlated in ways that depend on each laser system. To evaluate locking stability, two lasers are locked to adjacent modes of the same cavity. The beat signal between the two laser frequencies is then monitored for fluctuations. A relative locking performance of 1 part in 105 of the interferometer linewidth can be accomplished with a high finesse interferometer cavity having a resonance linewidth of less than 10 kHz, leading to relative stability between two lasers at the sub-Hz level for nominally a minute, when substantial effort is made to vibrationally and thermally isolate the cavity from its surrounding environment. The short-term drift of the frequency of either of these lasers arises from thermal length changes, mechanical creep, vibration, or other variation in the length of the reference cavity and hence resonance frequency. Attempts have been made to approach absolute stability using Fabry-Perot cavities made from sapphire and kept at cryogenic temperatures to achieve a 3 kHz drift over 6 months.
Atomic transitions are commonly used to lock a laser to a specific, absolute frequency. A typical arrangement would substitute a fixed atomic resonance for the cavity resonance and allow the modulated beam to transmit through a gas-phase sample before collecting it on a photodetector. Today""s precision clocks and oscillators are based on well-studied microwave transition frequencies of rubidium, cesium, hydrogen, and mercury atoms in oscillators and masers, carbon dioxide lasers, cold calcium-stabilized diode lasers, lasers stabilized to optical transitions of iodine, cesium, rubidium, mercury, and ytterbium among others. Optical frequency standards in the communications bands of 1.3 and 1.5 micron wavelength are also of interest. Among the principal advantages of existing microwave and optical standards are that the transition frequencies of an atom are unique to its type, i.e. predetermined by nature and that FM locking techniques typically select the center of those transitions with high precision. A difficulty is that when the frequencies are not exactly those of interest, they must be transferred through a potentially elaborate chain of precisely controlled optical and radio frequency (rf) synthesis, such as frequency doubling and mixing or parametric oscillation sums and differences. That complication may be simplified by use of new optical frequency combs based on femtosecond mode-locked lasers and nonlinear effects in optical fibers. Lastly, strong optical dipole transitions typically have spectral linewidths in the 10""s of MHz (megaHertz, i.e., 106 Hz)xe2x80x94not especially narrow for use as a precise frequency discriminator for laser locking, though examples of higher multi-pole moment transitions with narrower linewidths but weaker transitions do exist; one such transition is the 2S1/2xe2x88x922D5/2 electric-quadrupole transition of a single trapped 199Hg+ ion (wavelength of 282 nm and natural linewidth of 2 Hz). The present optimum realization of the 199Hg+ ion standard requires operation of a Paul ion trap at liquid helium temperatures.
Another frequency locking technique involves stabilization to a Lamb dip in a gas vapor cell. For the Lamb dip, the gaseous motion of the atoms produces a Doppler shifted distribution of frequencies creating an inhomogeneously broadened absorption-line. A strong laser, intense enough to saturate a particular velocity subset of these atoms can temporarily create a spectral hole anywhere in the absorption profile until it is turned off and the hole disappears (absorption reappears). A less intense probe laser can be locked to the hole created in the presence of the strong laser. The frequency stability of the Lamb dip spectral hole is determined by that of the strong laser. To remove this external stability dependence, the more typical arrangement is to use a single laser divided into two counter-propagating beams, strong and weak. For a lock to occur, the two beams must interact with the same velocity subset of atoms that must be the non-Doppler shifted subset. The lock is then constrained to the center of the inhomogeneous absorption.
The current US atomic frequency standard is the cesium-fountain standard, called NIST-F1. It uses laser-cooled atoms that are slowed to near absolute zero temperature (and hence near zero velocity) and then tossed vertically through a microwave cavity, after which the cooling lasers are turned off and the atoms return under the influence of gravity to a detector below the level of the microwave cavity. Because the atoms move at much lower speed than are found in the previous atomic beam standards, this standard has smaller systematic frequency shifts.
Condensed phase spectral hole burning materials are known. Absorption features of ions or molecules doped into condensed phase materials are spectrally broadened by two main classes of mechanisms. Homogeneous broadening is the fundamental broadening experienced by all ions or molecules independently, and arises from the quantum-mechanical relationship between the lineshape and the dephasing time of the excited ion. At cryogenic temperatures, such homogeneous linewidths have been measured, using the photon-echo technique, to be as narrow as 100 Hz or less, orders of magnitude sharper than most gas phase transitions. Inhomogeneous broadening, such as that of the inhomogeneous absorption profile 210 depicted in FIG. 2A, arises from the overlap of the quasi-continuum of individual spectra 220 of all of the ions or molecules in the condensed phase material, which have microscopically different environments and therefore slightly different transition frequencies. The extent of this envelope can be anywhere from hundreds of MHz to the THz (teraHertz, i.e., millions of MHz) range.
Persistent spectral hole burning can be accomplished by using a narrow-band laser to selectively excite only the small fraction of ions or molecules whose frequencies around a chosen central frequency 240 coincide with that of the laser. If some mechanism exists to remove those ions or molecules from the absorbing population, or to change their resonant frequencies, then the inhomogeneous absorption profile can be altered, leaving a xe2x80x9cspectral holexe2x80x9d 250 at the frequency of the laser, as seen in FIG. 2B depicting the altered inhomogeneous absorption profile 230. In the cases of interest here, the homogeneous linewidth 220xe2x80x2xe2x80x3 is many orders of magnitude smaller than the inhomogeneous linewidth 210, and by probing with a tunable laser in the vicinity of a spectral hole, a sharp positive transmission peak is observed on a broad background of partial transmission.
The use of persistent spectral hole burning for laser frequency stabilization is shown in FIG. 3. In laser frequency stabilization using persistent spectral hole burning, a Fabry-Perot cavity is eliminated from the stabilization system and instead a beam is transmitted through a persistent spectral hole burning material (SHBM). The output of a tunable laser 110 on path 360 is split at a beam splitter 320 before entering the modulator 112 along path 361. The modulator uses an input rf signal to modulate the laser output, as in FIG. 1A. The output from the modulator is directed along path 362 through a persistent SHBM 310. The persistent SHBM includes a persistent spectral hole that is at a desired output frequency in a narrow spectral width. The output from the persistent SHBM 310 is used as a reference beam, which is detected by the detector 120 which is like the detector 120 of FIG. 1A. In the case that the spectral hole burning materials are gated burning hole materials, the laser 120 does not cause any permanent change unless some other control conditions simultaneously exist, such as a control field generator 370. Over time, repeated hole burning in a persistent spectral hole burning material while frequency locking will cause the spectral holes to saturate and broaden, degrading lock stability. Further, there are relatively few persistent spectral hole burning materials.
It is known how to lock a mode-locked pulsed laser with a spectrum shown in FIG. 13, simultaneously to a large number (e.g 500,000) of adjacent longitudinal modes of a Fabry-Perot cavity to stabilize the frequency, phase, and repetition rate of its pulse train, as shown in FIG. 14. The crucial requirement is that all the error signals generated from all the reference cavity modes and all the pulsed laser modes (in one single laser beam and after being passed through the modulator are each individually phase-modulated with their own set of sidebands) be adjusted to constructively add to form a total error signal that looks nearly identical to the error signal for a single frequency laser. The error signal may then be applied through a servo amplifier to the appropriate tuning elements of the mode-locked laser, such as but not limited to: a piezoelectrically mounted mirror to vary the cavity length and a piezoelectrically controlled dispersive element such as a prism or tilting mirror to manage this dispersion. There are two principal difficulties in doing so: a) the mode spacings must match, and b) the absolute frequency of each spectral component in each comb must be aligned to overlap by tuning the laser pulse train spectrum to match those set by the Fabry-Perot reference cavity. This can only be done approximately when locking to a Fabry-Perot reference cavity. The time-domain pulse repetition of the mode-locked laser is very steady except for thermal drift of the laser cavity; this gives rise in the Fourier transformed frequency spectrum to a comb of extremely uniformly spaced modes. However, the mode spacings of the Fabry-Perot cavity do not nearly approach this uniformity. While they are all nominally spaced by the inverse of the round trip time for light traversing the cavity, dispersion (or variation with frequency) compresses or expands the mode spacings in the outer reaches of the reference cavity mode spectrum because the group velocity dispersion of the mirrors varies the optical path length of the cavity for the widely different frequency components. Hence, only some central modes are exactly overlapped and those much further away accrue an increasing degree of frequency mismatch. The composite error signal suffers to some degree from the mismatch.
The frequency locking achievable using Fabry-Perot cavities, as shown in FIG. 1A, is limited by the thermal length changes, mechanical creep, vibration and other variations in the length of the reference cavity influenced by external causes. The frequency locking achievable using atomic transitions are limited by available central frequencies associated with those transitions and the characteristics of the frequency reference such as the Lamb dip. The frequency locking achievable using persistent spectral hole burning materials is limited by the broadening of the spectral hole and the range of materials that can be used as the persistent spectral hole burning material. There is a need for a laser frequency stabilization technique providing better stabilization using a frequency reference with a consistently narrow bandwidth. There is also the need for a spectral hole burning technique to stabilize the frequency of a laser with many different material options for the spectral hole burning material.
An object of the present invention is to link together existing techniques of frequency locking and transient spectral hole burning to more accurately stabilize the frequency of a laser. Another object of the present invention is to use transient spectral hole burning materials as both laser stabilizers and signal processors.
The present invention relates to a device for actively stabilizing the frequency of a laser by locking to a transient spectral hole in a solid state reference material. A transient spectral hole may naturally have a lifetime up to a few tens of milliseconds. However, as long as the laser is continuously illuminating the transient spectral hole material and is stabilized to the center frequency of the hole, the spectral hole is continuously renewed, providing and maintaining a stabilizing reference. Many more materials exhibit transient spectral hole burning than do persistent spectral hole burning, giving an access to a greater range of available frequencies at which lasers can be stabilized. As a high level stabilization is achieved on millisecond time scales, the stabilization is ideal for providing sources for coherent transient effects, in particular, the photon echo and stimulated photon echo phenomenas that are the basis for a wide range of optical processors.
Techniques for stabilizing a laser at a selectable frequency include splitting an output beam from an electrically adjustable laser into a first beam and a second beam. The second beam is transmitted through a modulator to produce a modulated second beam. Then the modulated second beam is transmitted through a transient spectral hole burning material onto a detector. The laser is electrically adjusted in response to a detector output from the detector which senses the changes in the modulated second beam after it passes through the transient spectral hole burning material.
The transient hole burning frequency stabilizer has particular applications in devices that require extreme frequency stabilization on relatively brief time scales. This includes, but is not limited to, other devices based on the time and frequency-domain spectral hole burning, such as optical signal routers and switches, processors, correlators, true time delay generators, rf spectrum analyzers, and for high resolution spectroscopy.
Other applications include portable and transportable frequency references, precision laser ranging and long baseline interferometery, a high quality local oscillator component in frequency standard or clock system that provides an accurate absolute time and frequency reference for applications, a stabilization component in a frequency comb system or other frequency synthesizer, laser frequency and time standards for use in space for example to synchronize or control the positions of satellite networks, local oscillators for coherent communications, synchronization for advanced generations of telecommunications networks, optical source for quantum computation, and optical source for quantum-limited measurements on atoms.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.